Each HVM is implemented using a hypervisor: either KVM or bhyve. KVM was the first HVM supported and remains a strong choice for most VMs, particularly if you are already using KVM in Triton and are satisfied with the performance and features. Bhyve is a newer HVM which aims to improve performance and scalability, while growing the feature set. Because bhyve is relatively new, not all images are supported on bhyve.